The present invention relates to large diameter microwave waveguides, and more particularly to a distributed window that may be used in such waveguides to couple high frequency, high power microwave radiation through a vacuum barrier within the waveguide without overheating, significant mode conversion, or reflection of incident power.
A waveguide window in a microwave power system permits power to be coupled from a first waveguide to a second waveguide, but presents a physical barrier between the two waveguides. The physical barrier allows the waveguides to contain different gases or to be at different pressures, and one or both waveguides may be evacuated. For example, in high power microwave vacuum devices, such as gyrotrons and the like, the output power must be coupled between an evacuated chamber or waveguide in the gyrotron device, through one or more waveguide windows, into a waveguide having a gaseous environment. The one or more waveguide windows thus provide a hermetic seal between the two media. Also, in fusion reactors where microwave power is added to a plasma, the physical barrier of a microwave window may be placed near the reactor to confine the constituents of the plasma.
One type of microwave window known in the art is described in U.S. Pat. No. 5,061,912, incorporated herein by reference. A similar type of window is described in U.S. patent application Ser. No. 07/898,502; filed Jun. 15, 1992 now abandoned, also incorporated herein by reference. The types of microwave windows disclosed in the '912 patent and the '502 application are distributed windows that form part of a phase velocity coupler. The type of coupling provided by the described windows is between two identical corrugated rectangular waveguides, each of which is many (e.g.,&gt;15) free space wavelengths, .lambda..sub.0, wide in one transverse dimension but only 2 to 3 .lambda..sub.0 in the other dimension. A transition from circular corrugated waveguide many .lambda..sub.0 in diameter propagating the HE.sub.11 mode, which is a preferred method of low loss transmission for high power millimeter wavelength microwaves, to this rectangular corrugated waveguide, can always be made. However, if the circular waveguide is very large, e.g., 30.lambda..sub.0 in diameter, many modes which can propagate in the larger circular waveguide are cut off in the rectangular waveguide. Although ideally, only one mode is emitted from the source, typically a gyrotron, and propagated through the system, in reality there is often a few percent of other modes present, which might be reflected back to the source with deleterious effects by such a transition. Hence, there is a need in the art for a microwave window that can be used to directly and efficiently couple high frequency microwave power between two large diameter waveguides without the need for any transitions to other shapes and sizes.
There also exists a new generation of gyrotrons, such as the Russion 500 kw, 110 and 140 GHz gyrotrons which have the HE.sub.11 output mode, which are most compatible with a large output diameter. Unfortunately, a suitable CW vacuum window does not presently exist for such large diameters.
The present invention addresses the above and other needs.